Buried butted contact and method for fabricating

ABSTRACT

A buried butted contact and method for its fabrication are provided which includes a substrate having dopants of a first conductivity type and having shallow trench isolation. Dopants of a second conductivity type are located in the bottom of an opening in said substrate. Ohmic contact is provided between the dopants in the substrate and the low diffusivity dopants that is located on a side wall of the opening. The contact is a metal silicide, metal and/or metal alloy.

CROSS-REFERRENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.09/126,577 now U.S. Pat. No. 6,153,934 filed Jul. 30, 1998.

DESCRIPTION TECHNICAL FIELD

The present invention is concerned with a buried butted contact. Moreparticularly, the present invention is concerned with providing an ohmiccontact through shallow trench isolation of an n-type diffusion toground or a type diffusion to Vdd. The buried contact of the presentinvention is vertically displaced from the top surface of the substrateand permits the contact to be formed in reduced area. The buried contactof the present invention does not extend to any significant extent underthe active surface diffusion. According to the present invention, aburied diffusion is contacted below the surface of the substrate tosurface diffusion of an opposite polarity type by a metallic-typeinterconnection line. The metallic-type interconnection line can be ametal, a metal alloy, and/or an intermetallic silicide. The presentinvention is also concerned with a method for fabricating the buriedbutted contact.

BACKGROUND OF INVENTION

Butted contacts have been used for contacting n-type diffusion to groundor a p-type diffusion to Vdd power supply. Butted contacts provide arelatively dense contacting method. A method typically employed forcreating butted contacts involves placing opposite diffusion blockswithin the same active area, whereby the diffusion blocks do notoverlap. However, this technique requires sufficient real estate inorder to guarantee that both diffusion types are reliably created,thereby taking up increased area.

Furthermore, the substrate well contact diffusion, when butted against aFET diffusion, tends to create some degradation in transistorperformance as the well contact diffusion gets closer and approaches theFET gate edge. It is believed that dopant diffusion of opposite speciestype from the substrate or well contact causes this degradation.

Accordingly, it would be desirable to provide a butted contact thatrequires less real estate along with eliminating or at leastsignificantly reducing contamination of the device by dopant diffusionfrom the butted contact.

SUMMARY OF INVENTION

The present invention provides butted contacts that require reduced areaalong with eliminating contamination of the device from the dopants ofthe contact. The present invention provides for contacting an n-typediffusion to ground or a p-type diffusion to Vdd power supply. Accordingto the present invention, the contact is vertically displaced from thetop surface of the substrate.

More particularly, the present invention is concerned with a buriedbutted contact that comprises a substrate having dopants of a firstconductivity type and having shallow trench isolation. Dopants of asecond and opposite conductivity type are located in the bottom of anopening through the substrate and down to the bottom of the shallowtrench isolation. Ohmic contact between the dopants of the firstconductivity type and the dopants of the second and oppositeconductivity type is provided on a side wall of the opening. The ohmiccontact is a metallic type interconnect being a metal, metal alloyand/or intermetallic silicide. The opening overlaps the edge portion ofthe shallow trench isolation and overlaps a small portion of the dopantsof the first conductivity type.

The present invention is also concerned with a method for forming theabove-described buried contact. The method of the present inventioncomprises providing a substrate having dopants of a first conductivitytype and having shallow trench isolation. An opening is delineated inthe substrate down to the vicinity of the bottom of the shallow trenchisolation and overlaps a small portion of the edge of the shallow trenchisolation and a small portion of an edge of the dopants of the firstconductivity type. Dopants of a second and opposite conductivity typeare implanted into the bottom of the opening to thereby form a buriedcontact. A layer of a metallic type electrically conductive material isdeposited on a side wall of the opening to provide ohmic contact betweenthe dopants of the first conductivity type in the substrate and dopantsof the second and opposite conductivity type. The metallic-typeelectrically conductive material is a metal, metal alloy and/orintermetallic silicide.

Still other objects and advantages of the present invention will becomereadily apparent by those skilled in the art from the following detaileddescription, wherein it is shown and described only the preferredembodiments of the invention, simply by way of illustration of the bestmode contemplated of carrying out the invention. As will be realized theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious respects,without departing from the invention. Accordingly, the description is tobe regarded as illustrative in nature and not as restrictive.

SUMMARY OF DRAWINGS

FIGS. 1-2 are schematic illustrations of the structure at differentstages of fabrication according to the present invention.

BEST AND VARIOUS MODES FOR CARRYING OUT INVENTION

In order to facilitate an understanding of the present invention,reference will be made to the figures which illustrate a diagrammaticrepresentation of the steps of fabricating the buried butted contactaccording to the present invention.

It is to be understood that when the discussion refers to n-typeimpurities, the particular process steps are applicable to p-typeimpurities, and vice versa. Also, the present invention is applicable tosubstrates other than silicon which are known in the art such as othersemiconductor materials including group III-V semiconductors.Furthermore, when reference is made to impurities of a “first type” andto impurities of a “second type”, it is understood that the “first type”refers to n- or p-type impurities and “second type” refers to theopposite conductivity type. That is, if the “first type” is p, then the“second type” is n. If the “first type” is n, then the “second type” isp. P-type dopants for silicon include boron, aluminum, gallium, andindium. N-type dopants for silicon include arsenic, phosphorus andantimony.

According to the present invention, shallow trench isolation 2 isprovided in a semiconductor substrate 1 (see FIG. 1). The semiconductorsubstrate is typically silicon but can be any other semiconductormaterial such as group III-V semiconductor. The isolation can be silicondioxide. Also, in the vicinity of the top portion of substrate 1 isdopant 3 of a first conductivity type. The dopant for an n-type dopantin the case of silicon can be arsenic, phosphorus and antimony and thein the case of a p-type dopant can be boron, aluminum, gallium andindium. The dopant can be provided by any conventional technique such asdiffusion or ion implantation. Typically, the dopant concentration ofthe first conductivity type is at least about 5×10¹⁹ atoms/cm³.

An opening 4 is provided in the substrate by typical patterningtechniques such as by applying a photosensitive resist material,exposing it imagewise to actinic light such as UV radiation, X-rayradiation or E-beam radiation in a predetermined pattern to define theopening. In the case of a positive resist, the portion exposed to theactinic light is removed by dissolving in a suitable solvent. In thecase of a negative photoresist, the portion not exposed to the actiniclight is removed by dissolution in a suitable solvent. The patternedphotoresist then acts as a mask for removing exposed portions of thesubstrate down to within the vicinity of the bottom of the shallowtrench isolation. The opening is patterned such that the openingoverlaps a small portion of the shallow trench isolation at the trenchedge and a small portion, typically about 0.01 μm² to about 1 μm² of thediffusion 3 of the first conductivity type. In other words, only a smallportion of the opening overlaps the edge of the shallow trench isolationextend past the edge of the conductivity of the first conductive typebut does extend to the edge of such. In the case of a PFET buttedcontact, the conductivity of the first type will be an n-well and for aNFET butted contact, the conductivity of the first type will be ap-well.

The portions of substrate 1 not protected by the photoresist mask can beremoved such as by employing reactive ion etching.

In a typical example according to the present invention, a dopant 5 ofthe second and opposite conductivity type is then implanted into thebottom of the opening 4. In the case of the dopant of the first typebeing n+, the preferred dopant of the second type (p-type) is indium. Inthe case of the dopant of the first type being p-type, the preferreddopant of the second type (n-type) is antimony. According to preferredaspects of the present invention, the dopant 5 of the second andopposite conductivity type exhibit relatively low diffusivitycharacteristics. The dosage of the dopant of the second and oppositeconductivity type should be sufficient to provide an adequate ohmiccontact and is typically at least about 1×10¹⁴ atoms/cm² and moretypically the dosage is no greater than about 1×10¹⁶ atoms/cm².

A second photoresist 6 is applied and then patterned to expose only theareas to provide the butted contact. The pattern exposes a portion ofthe shallow trench isolation at the boundary with the silicon substrate.The exposed portion of oxide isolation is removed by etching down to thebottom of the substrate. Typically, the small portion is only about 0.01μm² to about 1 μm², and more typically about 0.03 μm² to about 0.09 μm².

Next, the metallic type interconnection 7 (see FIG. 2) between thedopants of the first conductivity type and dopant of the secondconductivity type is formed. In particular, examples of some suitablemetal silicide layers include titanium silicide, molybdenum silicide,zirconium silicide, hafnium silicide, vanadium silicide, niobiumsilicide, tantalum silicide, chromium silicide, and tungsten silicide.Such can be formed by sputtering such as from a solid silicide target.In addition, the metallic-type interconnect can be provided bydepositing a layer of a refractory metal or refractory metal alloy suchas tungsten, titanium, zirconium, hafnium, vanadium, niobium, tantalum,chromium, and molybdenum. The preferred metal is tungsten. Such can bedeposited by sputtering or chemical vapor deposition. The methods fordepositing the interconnect layers are well known in the art and neednot be described herein in any great detail. For instance, see U.S. Pat.Nos. 3,768,150 and 4,392,150, disclosures of which are incorporatedherein by reference.

The metallic-type interconnect layer 7 is provided on side wall of theopening and contacts the diffusion of the second type and extends to thediffusion of the first type. It is not necessary that the metallic typeinterconnect extend continuous over the entire area of the dopant of thesecond conductivity type. The silicide interconnect layer is about 200to about 1000 Å thick and preferably about 300 to about 500 Å thick.

The structure subjected to further processing to complete the device asreadily apparent to those skilled in the art.

As can be appreciated from the above description, the contact createdaccording to the present invention is vertically displaced from thesurface, thereby allowing it to be formed in much smaller area. Inparticular, a mask to block implants of one conductivity type need notbe defined adjacent to a mask for blocking impurities of the secondconductivity type so as to guarantee formation of the opposite polarityjunction. Furthermore, the present invention provides a buried contactimplant that is self-aligned to the opposite polarity doped substrateand that no dopant contamination of the device occurs from the buttedburied contact.

The foregoing description of the invention illustrates and describes thepresent invention. Additionally, the disclosure shows and describes onlythe preferred embodiments of the invention but, as mentioned above, itis to be understood that the invention is capable of use in variousother combinations, modifications, and environments and is capable ofchanges or modifications within the scope of the inventive concept asexpressed herein, commensurate with the above teachings and/or the skillor knowledge of the relevant art. The embodiments described hereinaboveare further intended to explain best modes known of practicing theinvention and to enable others skilled in the art to utilize theinvention in such, or other, embodiments and with the variousmodifications required by the particular applications or uses of theinvention. Accordingly, the description is not intended to limit theinvention to the form disclosed herein. Also, it is intended that theappended claims be construed to include alternative embodiments.

What is claimed is:
 1. A method of forming a buried contact whichcomprises: providing a substrate having dopants of a first conductivitytype and having shallow trench isolation; delineating an opening in saidshallow trench isolation down to said substrate; implanting dopants of asecond and opposite conductivity type into the bottom of said opening tothereby form a buried contact; and depositing a layer of an electricallyconductive material selected from the group consisting of metalsilicide, metal, metal alloy and mixtures thereof on a side wall of saidopening to thereby provide ohmic contact between said dopants in saidsubstrate and second dopants.
 2. The process of claim 1 wherein saidelectrically conductive material layer comprises a metal.
 3. The processof claim 2 wherein said metal is a refractory metal.
 4. The process ofclaim 2 wherein said metal is tungsten.
 5. The process of claim 1wherein said electrically conductive material layer comprises a metalalloy.
 6. The process of claim 1 wherein said electrically conductivematerial layer comprises a metal silicide.
 7. The process of claim 1wherein the concentration of the dopant of the first conductivity typeis at least about 5×10¹⁹ atoms/cm³.
 8. The process of claim 1 whereinthe dosage of the dopant of the second and opposite conductivity type isat least about 1×10¹⁴ atoms/cm².
 9. The process of claim 1 wherein saidsubstrate is silicon.
 10. The process of claim 9 which comprises iridiumas one of the dopants and antimony as the dopant of the opposite type.